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Vitronectin specific cell receptor derived from mammalian mesenchymal tissue |
| 4789734 |
Vitronectin specific cell receptor derived from mammalian mesenchymal tissue
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| Patent Drawings: | |
| Inventor: |
Pierschbacher |
| Date Issued: |
December 6, 1988 |
| Application: |
06/763,046 |
| Filed: |
August 6, 1985 |
| Inventors: |
Pierschbacher; Michael D. (San Diego, CA)
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| Assignee: |
La Jolla Cancer Research Foundation (La Jolla, CA) |
| Primary Examiner: |
Schain; Howard E. |
| Assistant Examiner: |
Kushman; Jeff P. |
| Attorney Or Agent: |
Pretty, Schroeder Brueggemann & Clark |
| U.S. Class: |
210/635; 530/350; 530/380; 530/395; 530/413; 530/840; 530/841; 530/842 |
| Field Of Search: |
530/395; 530/356 |
| International Class: |
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| U.S Patent Documents: |
4356117; 4431582 |
| Foreign Patent Documents: |
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| Other References: |
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Isolation of a Laminin-Binding Protein from Muscle Cell Membranes, Herve Lesot et al., The EMBO Journal, vol. 2, No. 6, 1983..
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Concomitant Loss of Cell Surface Fibronectin and Laminin from Transformed Rat Kidney Cells, Edward Hayman et al., J. of Cell Biology, vol. 88, 1981..
Serum Spreading Factor (vitronectin) is present at the Cell Surface and in Tissues, Edward Hayman et al., Proc. Natl. Acad. Sci., vol. 80, 1983..
Cell Attachment Activity of Fibronectin Can Be Duplicated by Small Synthetic Fragments of the Molecule, M. D. Pierschbacher et al., Nature, vol. 309, 1984..
Variants of the Cell Recognition Site of Fibronectin that Retain Attachment-Promoting Activity, M. D. Pierschbacher et al., Proc. Natl. Acad. Sci., vol. 81, 1984..
Location of the Cell Attachment Site in Fibronectin with Monoclonal Antibodies and Proteolytic Fragments of the Molecule, M. D. Pierschbacher et al., Cell, vol. 26, 1981..
The Cell Attachment Domain of Fibronectin, M. D. Pierschbacher et al., J. of Biological Chemistry, vol. 257, 1982..
Alignment of Biologically Active Domains in the Fibronectin Molecule, Erkki Ruoslahti et al., J. of Biological Chemistry, vol. 256, 1981..
Interaction of Soluble Fibroblast Surface Antigen with Fibrinogen and Fibrin, Erkki Ruoslahti et al., J. of Experimental Medicine, vol. 141, 1975..
Fibronectin: Current Concepts of it's Structure and Functions, Erkki Ruoslahti et al., Doll. Res., vol. 1/1981..
Localiation of the Gangliosides GD2 and GD3 in Adhesion Plaques and On the Surface of Human Melanoma Cells, D. A. Cheresh et al., Proc. Natl. Acad. Sci., vol. 81, 1984..
Inhibition of Human Melanoma Cell Growth in Vitro by Monoclonal Anti-GD3-Ganglioside Antibody, W. G. Dippold et al., Cancer Research 44, Feb. 1984..
Chemical Characterization of a Neural Cell Adhesion Molecule Purified from Embryonic Brain Membranes, S. Hoffman et al., J. of Biological Chemistry, vol. 257, 1982..
Antisera Inhibiting Mammalian Cell Spreading and Possible Cell Surface Antigens Involved, P. Hsieh et al., J. of Cell Biology 1980..
Membrane Glycoproteins Involved in Cell-Substratum Adhesion, K. A. Knudsen et al., Proc. Natl. Acad. Sci., vol. 78, 1981..
Antibody Affinity May Influence Antigenic Modulation of the Common Acute Lymphoblastic Leukemia Antigen in Vitro, T. W. Lebien et al., J. of Immunology 1982..
Binding of Plasma Fibronectin to Cell Layers of Human Fibroblasts, P. J. McKeown-Longo et al., J. of Cell Biology 1983..
New Surface Component of Fibroblast's Focal Contacts Identified by a Monoclonal Antibody, B. Gresch et al., Cell, vol. 31, 1982..
Inhibition of Fibronectin Receptor Function by Antibodies Against Baby Hamster Kidney Cell Wheat Germ Agglutinin Receptors, N. Oppenheimer et al., J. of Cell Biology, vol. 95, 1982..
Studies of Cell Adhesion and Recognition 1. Extent and Specificity of Cell Adhesion Triggered by Carbohydrate-Reactive Proteins (Clycosidases and Lectins) and by Fibronectin, H. Rauvala, J. of Cell Biology, vol. 88, Jan 1981..
Teratocarcinoma Cell Adhesion: Identification of a Cell-Surface Protein Involved in Calcium-Dependent Cell Aggregation, C. Yoshiko et al., Cell, vol. 28, 1982..
Formation of Stress Fibres and Focal Adhesion Sites in Monensin-Exposed Cultured Human Fibroblasts in Response to Exogenously Added Cellular Fibronectin, V. P. Lehto et al., 1985..
Platelet Membrane Glycoprotein IIb-IIa: Member of a Family of Arg-Gly-Asp-Specific Adhesion Receptors, R. Pytela, et al., Complete Amino Acid Sequence of Human Vitronectin Deduced From cDNA Similarity of Cell Attachment Sites in Vitronectin andFibronectin, S. Suzuki et al., The EMBO Journal, vol. 4, 1985..
Detachment of Cells from Culture Substrate by Soluble Fibronectin Peptides, E. G. Hayman et al., J. of Cell Biology 1985..
A 125/115 kDa Cell Surface Receptor Specific for Vitronectin Interacts with the Arginine-Glycine-Aspartic Acid Adhesion Sequence Derived from Fibronectin, R. Pytella et al., Proc. Natl. Acad. Sci., vol. 82, 1985..
Adhesion of Platelets to Laminin in the Absence of Activation, C. R. Ill et al., J. of Cell Biology, vol. 99, 1984..
Inhibition of Fibronectin Binding to Platelets by Proteolytic Fragments and Synthetic Peptides Which Support Fibroblast Adhesion, M. Ginsberg et al., J. of Biological Chemistry 1985..
The Effect of Arg-Gly-Asp-Containing Peptides on Fibrinogen and von Willebrand Factor Binding to Platelets, E. Plow, et al., Proc. Natl. Acad. Sci., vol. 82, 1985..
Cloning and Characterization of Two cDNAs Coding for Human von Willebrand Factor, J. Evan Sadler et al., Proc. Natl. Acad. Sci., vol. 82, 1985..
Effects of Fibronectin-Related Peptides on Cell Spreading, J. Sllnutze et al., In Vitro Cellular & Developmental Biology, vol. 21, 1985..
The Amino Acid Sequence of the cx-Chain of Human Fibrinogen, R. F. Doolittle et al., Nature, vol. 280, 1979..
Analysis of Platelet Adhesion With a Radioactive Chemical Cross Linking Reagent: Interation of Thrombospondin with Fibronectin and Collagen, J. Lanav et al., Cell, vol. 31, 1982..
Platelet Membrane Defects in Glanzmann's Thrombasthenia, D. R. Phillips et al., J. of Clinical Investigation, vol. 60, 1977..
Inhibition of von Willebrand Factor-Platelet Interaction by Fibrinogen, G. Pietu et al., Nature, vol. 308;984..
Evidence that Three Adhesive Proteins Interact with a Common Recognition Site on Activated Platelets, E. Plow et al., J. of Biological Chemistry, vol. 259, 1984..
Platelets Have More Than One Binding Site for von Willebrand Factor, Z. Buggeri et al., J. Clin. Investigation, vol. 72, 1983..
Glanzmann Thrombasthenia: Deficient Binding of von Willebrand Factor to Thrombin-Stimulated Platelets, Z. Ruggeri et al., Proc. Natl. Acad. Sci., vol. 79, 1982..
Platelet-Collagen Adhesion: Inhibition by a Monoclonal Antibody that Binds Glycoprotein IIB, P. J. Shadle et al., J of Cell Biology, vol. 99, 1984..
Identification of Two Structurally and Functionally Distinct Sites On Human Platelet Membrane Glycoprotein IIb-IIa Using Monoclonal Antibodies, R. P. McEver, J. of Biological Chemistry, vol. 258, 1983..
Purification of Glycoproteins IIb and IIl from Human Platelet Plasma Membranes and Characterization of a Calcium-Dependent Glycoprotein IIb-IIl Complex, L. K. Jennings et al., J. of Biological Chemistry, vol. 257, 1982..
Detachment of Cells From Culture Substrate by Soluble Fibronectin Peptides, E. G. Hayman et al., J. of Cell Biology, vol. 100, 1985..
Interction of Ap-2, a Monoclonal Antibody Specific for the Human Platelet Glycoprotein IIb-IIa Complex with Intact Platelets, D. Pidard et al., J. of Biological Chemistry, vol. 258, 1983..
ADP-Sependent Common Receptor Mechanism for Binding of von Willebrand Factor and Fibrinogen to Human Platelets, S. Timmons, et al. Proc. Natl. Acad. Sci., vol. 81, 1984..
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Secreted Alpha Granule Proteins, D. F. Mosher et al., 1985..
Interaction of Purified Type IIb von Willebrand Factor with the Platelet Membrane Glycoprotein Ib induces Fibrinogen Binding to the Glycoprotein IIb/IIIa Complex and Initiates Aggregation, L. DeMarco, Proc. Natl. Acad Sci., vol. 82, 1985..
Loss of Fibrinogen Receptors from the Platelet Surface During Simulated Extracorporeal Circulation, J. Musial et al., J. of Laboratory and Clinical Medicine, vol. 105, 1985..
A Murine Monoclonal Antibody That Completely Blocks the Binding Of Fibrinogen to Platelets Products a Thrombasthenic-like State in Normal Platelets and Binds to Glycoprotines IIb and/or IIIa, B. S. Coller et al., J. Clin. Invest., vol. 72 (1983)..
Reconstitution of the Purified Platelet Fibrinogen Receptor, L. V. Parise et al., J. of Biological Chemistry, 1985..
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| Abstract: |
A method of isolating cell surface receptors utilizing a short peptide sequence bound to an affinity column. Cell surface receptors which bind selectively to the short peptide and which are specific to various adhesion proteins may be isolated therewith from various cell preparations. These receptors, whose functional integrity has been maintained by the presence of the peptide ligand, are incorporated into liposomes and used to deliver specific compounds inside the liposomes to select tissues containing the specific adhesion proteins. |
| Claim: |
We claim:
1. A substantially purified cell surface receptor derived from mesenchymal tissue and capable of binding to a peptide containing the amino acid sequence Arg-Gly-Asp, comprising aglycoprotein composed of at least two polypeptides of about 115 and 125 kD, respectively, as determined by SDS-PAGE under reducing conditions which selectively binds to vitronectin, but not to fibronectin. |
| Description: |
BACKGROUND OF THE INVENTION
This invention relates generally to the field of cell biology and more specifically to cell adhesion systems.
Multicellular organisms, such as man, have some 10.sup.14 cells which may be divided into a minimum of fifty types, such as blood cells and nerve cells, etc. During the course of growth and development, cells adhere to other cells, or toextra-cellular materials, in specific and orderly ways. Such cell adhesion appears to be of importance in mediating patterns of cellular growth, migration and differentiation, whereby cells develop specialized characteristics, so as to function as, forexample, muscle cells or liver cells. Cell adhesion mechanisms also appear to be implicated in dedifferentiation, notably where cells lose their specialized forms and become metastasizing cancer cells.
The adhesion of cells to other cells or extracellular substrates appears to be at least a two step process: a cell must first recognize an appropriate site for attachment and thence selectively attach to it. Certain molecules have beenidentified which promote the attachment of cells to a solid substrate. Among these are fibronectin and vitronectin, two glycoproteins which are found in human plasma and serum. Unequal concentrations of specific adhesion promoting proteins are presentin various tissues, indicating that their functions may be somewhat different.
Adhesive proteins can be used, for example, to coat the surfaces of containers used for growing cells in tissue culture so as to promote the attachment of such cells. Conversely, by suspending such molecules in the culture medium, the cells maybe discouraged from attaching to the substrate. Because various types of cells will only grow effectively when attached to a solid substrate, the regulation of attachment is important in enabling the culturing of cells.
To gain further insight into the mechanisms of cell adhesion so as to permit manipulation and control, an understanding of the recognition sites on the cells themselves is necessary. Recently, a cell surface receptor specific to fibronectin hasbeen isolated and described (Pytela, et al., Cell 40:191, (1985)). This fibronectin receptor appears to specifically recognize and bind a portion of the fibronectin molecule which is composed of the amino acid sequence arginine-glycine-aspartic acid(Arg-Gly-Asp). Upon introducing fibronectin receptors to a solid substrate to which is attached fibronectin, the receptors will selectively bind the protein and can thus be manipulated.
Because of the critical role of cell surface receptors in regulating cell growth and differentiation both in vivo and in vitro, there exists a need for the isolation and purification of additional cell surface receptors. Specifically, there is aneed for cell surface receptors which may be specific to certain cell types, or exist in greater porportion in certain cell types, than the previously recognized fibronectin receptors. Further there exists a need to identify cell surface receptors whichcan be selectively manipulated with small and easily obtained molecules. Moreover, there exists a need for a method to protect the functional integrity of cell surface receptors during such manipulations. The present invention satisfies these needs andprovides additional advantages as well.
SUMMARY OF THE INVENTION
The present invention provides a novel method of isolating cell surface receptors utilizing a short peptide sequence bound to an affinity column. Cell surface receptors which bind selectively to the peptide and which are specific to variousadhesion proteins are isolated from various cell preparations. Cell surface receptors may be incorporated into the surface membranes of liposomes thereby targeting the liposomes to certain tissues. Moreover, such liposomes incorporating the cellsurface receptors may be used to deliver specific compounds contained inside the liposomes selectively to tissues exhibiting the specific adhesion proteins.
One aspect of the present invention is the isolation from osteosarcoma or other mesenchymal cells of a receptor molecule which specifically interacts with vitronectin. The receptor is composed of at least two polypeptides having apparentmolecular weights of 115 and 125 kilo Daltons (kD) as determined by SDS-PAGE under reducing conditions. The polypeptides are accessible to lactoperoxidase-catalyzed iodination at the cell surface. When incorporated into the membranes of liposomes,these polypeptides mediate the specific binding of the liposomes to vitronectin, but not to fibronectin. Moreover, antibodies raised against the vitronectin receptor do not detectably react with another cell surface receptor, the fibronectin receptor.
Another aspect of the invention is the isolation from platelet cells of a receptor which recognizes not only the extracellular matrices fibronectin and vitronectin, but the blood clot factor fibrinogen, and apparently von Willebrand factor aswell. Thus, this cell surface receptor has elements of a universal receptor binding to tissue components, including both extracellular matrices and blood clot factors.
In another aspect of the present invention, the cell surface receptors are incorporated into the membranes of liposomes by subjecting a detergent solution containing the receptor polypeptides and a phospholipid to dialysis against adetergent-free buffer. Such liposomes are then used to target the contents of the liposomes to tissues containing various molecules. The vitronectin receptor liposomes are useful in targeting liposomes in tissues rich in connective tissue. Forinstance, fibrosis of various organs may be treated with liposomes loaded with connective tissue-degrading enzymes using receptor-mediated targeting. Moreover, because many tumors contain large amounts of connective tissue, drugs may be delivered totumors with some degree of selectivity using receptor liposomes. The platelet receptor may be useful in targeting liposomes to intravascular blood clots such as those causing myocardial infarction as blood clots contain fibrin and fibronectin, both ofwhich can serve as ligands for the platelet receptor, and liposomes containing this receptor at their surface can be expected to bind specifically to blood clots. Agents designed to dissolve the clot could thus be delivered specifically to the clotitself. Liposomes containing platelet receptor or one of the other receptors may also be used to compete with the adhesion of platelets to fibrin clots or vascular walls in which the extracellular matrix has become exposed as a result of loss of theendothelial cell lining.
In another aspect of the invention, the cell surface receptors are protected by the continuous presence of their respective ligands, such as a peptide. Such protection results in the maintenance of the functional integrity of the cell receptoritself, thereby permitting use of the receptor, for example, in liposomes.
Other features and advantages of the present invention will become apparent from the following detailed description which illustrates, by way of example, the principles of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention involves a novel method of isolating cell surface receptors, certain isolated cell surface receptors, the protection of their functional integrity, their incorporation into liposomes and the use thereof to target therapeuticagents to specific tissues.
Cell surface receptors are isolated by utilizing an affinity column to which is bound a short peptide sequence. Receptors which bind to the peptide and which are specific to various adhesion proteins are isolated from various tissuepreparations, such as mesenchymal cells and platelets. These receptors may be incorporated into the surface membranes of liposomes so as to target the liposomes to certain tissue components, such as extracellular matrix and certain cells, therebyproviding a mechanism for delivering the contents of the liposomes to tissues exhibiting the specific adhesion proteins.
Certain cell surface receptors are isolated through the use of a column packed with Sepharose.RTM. to which is attached a short peptide. This peptide contains the amino acid sequence Arg-Gly-Asp, which is the binding site of fibronectin. Byapplying cell extract preparations of different cells to the column, cell surface receptors contained in the preparations are bound to the column. The receptors are then selectively eluted by flushing the column with a solution containing the peptide. The presence of the ligand protects the receptor, thereby maintaining its functionality.
One cell surface receptor so isolated is specific to vitronectin, and can be isolated from mesenchymal cells, such as osteosarcoma cells. Vitronectin, also called serum spreading factor, is a glycoprotein derived from human serum and plasmawhich can effect adhesion between certain cells and substrates. The active component of vitronectin is a protein composed of two closely related polypeptides having masses of 65 and 75 kD, respectively. Insoluble vitronectin is found in humanconnective tissue. The receptor comprises at least two polypeptides having masses of 115 and 125 kD.
Platelets are a class of cell-like structures found in the blood which exhibit complex adhesive interactions, notably aggregation of platelets and adhesion to extra cellular substrata. The cell surface receptor isolated from plateletpreparations by means of the Sepharose.RTM.-peptide column binds not only to fibronectin, but to vitronectin, fibrinogen and apparently von Willebrand factor as well. This receptor, therefore, appears to be a very generalized receptor.
Cell surface receptors can be incorporated into liposomes. In such a combination the receptors confer a specificity to the liposomes which permits them to deliver therapeutic agents to targeted tissues. A unique feature of the receptorliposomes is that rather than binding to cells, which targeted liposomes are usually designed to do, they will have affinity to extracellular matrices or fibrin and fibronectin in blood clots. When coated onto the surface of prosthetic materials, thereceptors would facilitate the bonding of a prosthesis, such as artificial skin, with extracellular matrix.
EXAMPLE I
Sepharose.RTM.-Peptide Column
A synthetic peptide having the amino acid sequence Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) (prepared to specification by Peninsula Laboratories, San Carlos, CA) is coupled to a cyanogen bromide activated agarose affinity matrix such as Sepharose.RTM. by the method of Axen, et al., Nature 214:1302 (1967).
EXAMPLE II
Isolation of the Vitronectin Receptor
MG-63 osteosarcoma cells were grown on 175 cm.sup.2 tissue culture dishes in Dulbecco's modified Eagle's medium (DMEM) supplemented with 5% fetal calf serum, glutamine and penicillin/streptomycin. For subculturing and harvesting, confluentlayers of cells were incubated in 1 mM ethylene diamintetraacetic acid (EDTA) for 15 minutes. For surface iodination, cells were harvested from confluent cultures, collected by centrifugation and resuspended in phosphate buffered saline (150 mM NaCl, 10mM sodium phosphate, 1 mM CaCl.sub.2, 1 mM MgCl.sub.2, pH 7.3) containing 0.2 mM PMSF. The suspended cells were radioiodinated with Na.sup.125 I according to Lebien at al., J. Immunol, 129:2287 (1982), which is incorporated by reference, and lysed in200 mM octyl-.beta.-D-glucopyranoside (octylglucoside, Behring Diagnostics, La Jolla, Calif.).
An octylglucoside extract containing 10.sup.8 cells in 1 ml mas applied to the affinity matrix which was then eluted with buffer containing detergent and the synthetic peptide, Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) which is a cell attachment-promotingpeptide including the fibronectin attachment sequence. The elution was carried out by slowly washing the column with 1 volume of column buffer supplemented with 1 mg/ml (1.5 mM) of GRGDSP over a period of one hour. The elution released two polypeptidesfrom the column having apparent molecular weights of 115 and 125 kD as determined by SDS-PAGE under reducing conditions as described in Laemmli, Nature 227:680 (1970). Samples were boiled for 3 minutes in the presence of 3% SDS, with or without 5%2-mercaptoethanol, and electrophoresed on 7.5% acrylamide gels. The molecular weight markers used were myosin (200 kD), 62 -galactosidase (116 kD), phosphorylase B (94 kD) and ovalbumin (43 kD). Gels were silver-stained using a reagent kit (BioRad,Richmond, Calif.), according to manufacturer's instructions. Autoradiography was performed using Kodak XAR X-Ray film placed between the dried gel and a Cronex Lightning Plus intensifying screen (DuPont, Newton, Conn.) and was continued for 1 to 3 daysat -70.degree. C.
EXAMPLE III
Alternative Isolation of the Vitronectin Receptor
Larger quantities of the receptor in an electrophoretically homogeneous form were obtained from human placenta. Two hundred grams of fresh-frozen human placental tissue were extracted with 200 ml octylglucoside or octylthioglucoside solution. The resulting extract was applied on a 10 ml peptide-Sepharose.RTM. column prepared and used as described above. The partially purified receptor obtained from the peptide column was further fractionated on a column of 1 ml wheat germagglutinin-Sepharose.RTM. (Pharmacia Fine Chemicals, Piscataway, N.J.), equilibrated with column buffer containing 0.1% NP-40 in the phosphate buffer saline detailed above. The receptor was eluted with 200 mM N-acetyl-glucosamine in the column buffer. The column fractions were analyzed by SDS-PAGE as described above, but the gell was stained with Coomassie blue.
EXAMPLE IV
Isolation of Platelet Receptor
Outdated human platelets (5 units) were harvested by centrifugation at 2200 rpm. The pellet was resuspended in Tyrode's buffer containing 1 mM CaCl.sub.2, and platelets were again collected by centrifugation. Five ml of cold 10 mM phosphate,150 mM NaCl.sub.2, 1 mM MgCl.sub.2, pH 7.3 (PBS), containing 50 mM octylthioglucoside (Behring Diagnostics, La Jolla, Calif.) and 3 mM PMSF were added to this pellet (5 ml). After resuspension of the pellet and incubation at 4.degree. C. for 15 min.,the lysate was centrifuged at 30,000 xg for 20 min. (4.degree. C.), and the supernatant (5 ml) was applied to a GRGDSP-Sepharose.RTM. column (bed volume 10 ml). the affinity matrix had been prepared by incubating 120 mg of GRGDSP(K)-peptide (thelysine residue (K) was added to optimize coupling) with 10 ml of CNBr-Sepharose.RTM. following the manufacturer's instructions. The extract was incubated with the affinity matrix overnight at 4.degree. C., and the column was washed with 50 ml of PBScontaining 25 mM octylthioglucoside and 1 mM PMSF. Elution of specifically bound components was accomplished by washing the column with 10 ml of column buffer containing 1 mg/ml of GRGDSP-peptide, and then with another 10 ml of column buffer. Finally,the column was washed with 8 M urea, 50 mM tris-HCl, pH 7.5, to release non-specifically bound proteins. Fractions (2.5 ml) were collected after the peptide solution was applied. Sixty .mu.l-aliquots of each fraction were mixed with 20 .mu.l of 12%SDS, 20% 2-mercaptoethanol, 60 mM Tris-HCl, pH 6.8, boiled for 3 min., and electrophoresed on a 7.5% polyacrylamide gel according to the method of Laemmli, above. Protein bands were visualized by staining with Coomassie blue. The amount of proteinrecovered from the column was about 0.5 mg, as estimated by densitometric scanning of the gel bands.
EXAMPLE V
Incorporation of Vitronectin Receptor Into Liposomes
Phosphatidylcholine liposomes incorporating the cell surface receptors were prepared essentially by the method of Mimms et al., Biochemistry 20:883 (1981), which is incorporated by reference. Egg yolk phosphatidylcholine (Sigma, St. Louis,Mo.), .sup.3 H-phosphatidycholine (New England Nuclear, Boston, Mass.) and the appropriate radioiodinated cell surface receptor fractions were dissolved in column buffer. This solution was then dialyzed against a detergent-free buffer, PBS for 24 hoursat 4.degree. C., resulting in the formation of liposomes with an average diameter of approximately 200 nm, as judged by electron microscopy. In the case of the 115 and 125 kD polypeptides of the vitronectin receptor, about 90% of the proteinradioactivity was found associated with the liposome fraction after the liposomes had been floated to the surface of a sucrose gradient by ultracentrifugation.
EXAMPLE VI
Binding of Vitronectin Receptor-Liposomes to Vitronectin and Peptide Substrates
Liposomes incorporating the radioiodine labelled vitronectin receptor polypeptides were used to study the recognition specificity of the receptor. Polystyrene Microtiter plate wells (Linbro/Titertek, Inglewood, Calif.) were coated withextracellular matrix proteins and peptides including vitronectin, fibronectin, laminin and the synthetic peptides Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) and Gly-Arg-Gly-Glu-Ser-Pro (GRGESP). The vitronectin receptor-liposome preparation showed dose dependentbinding to microtiter wells coated with vitronectin, but not fibronectin or laminin. Moreover, it bound to the GRGDSP peptide which contains the Arg-Gly-Asp sequence, but not the GRGESP peptide, which does not. The Arg-Gly-Asp sequence is thereforeimplicated in the binding of the liposomes containing the vitronectin receptor.
EXAMPLE VII
Use of the Receptor-Liposome Preparation to Target Therapeutic Agents
Receptor liposomes are prepared as described above and a desired therapeutic agent is incorporated into them according to published methods (Hashimoto et al., Cancer Research 43:5328 (1983); Gregoriadis and Senior, Biochem. Soc. Trans. 12:337(1984)) which are incorporated by reference. These conjugated liposomes are injected intravenously using 1-300 .mu.mol of lipid per kg of body weight or applied locally. Such vitronectin receptor liposomes are used, for example, to target tissuedegrading enzymes to tumors, which contain large amounts of connective tissue. Moreover, platelet receptor conjugated liposomes are used, for example, to deliver clot dissolving agents specifically to blood clots in the body, such as those causing amyocardial infarction.
EXAMPLE VIII
Use of the Receptor as a Coating on a Substrate
To coat a nonlipid surface with receptors, a receptor from a solution is covalently coupled to a surface such as plastic using one of the many well known methods available for such coupling. Receptor fragments that retain the ligand-bindingactivity but lack the membrane embedded portion of the molecule are advantageously used as they are more soluble than the complete receptor. Such materials coated with receptors are useful as prostheses where attachment of extracellular matrix isdesired, such as, for example, artificial skin.
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